The present invention relates to an induction heating type fixing device for an electrophotographic printer, facsimile apparatus, copier or similar image forming apparatus. More particularly, the present invention relates to a method of producing an induced current generating member for an induction heating type fixing device and exhibiting a desirable cooling effect. The present invention is applicable even to an induction coil for induction heating equipment in general.
An image forming apparatus of the type using toner for forming a visible image includes a fixing device for fixing a toner image formed on a paper or similar recording medium. Generally, the fixing device includes a heat roller for melting the toner with heat and a press roller pressing the paper against the heat roller while conveying it. The heat roller has thereinside a heater usually implemented by a halogen lamp. The halogen lamp heats the heat roller to a preselected fixing temperature.
However, the conventional heating system using a heater has the following left unsolved. It takes a substantial period of time for the heat roller to reach the fixing temperature (warm-up time), forcing the operator to simply wait without operating the apparatus at all. Another problem is that the heater implemented by a halogen lamp involves a substantial loss and therefore consumes substantial power. This is contrary to the current trend toward energy saving which is highlighted in the environment aspect. In such circumstances, there is an increasing demand for a fixing device featuring high efficiency and short warm-up time.
In light of the above, a fixing method of the kind heating a heat roller formed of conductive metal with an eddy current derived from an electromagnetic wave is attracting increasing attention. This kind of method, i.e., induction heating type fixing method drastically reduces the warm-up time and enhances efficiency and thereby contributes to the solution of environmental problems. One of conventional heat rollers for implementing this fixing method includes an induction coil wound spirally round a bobbin. When a high frequency current is caused to flow through the induction coil adjoining the inner periphery of the heat roller, a high frequency magnetic field is formed and induces an eddy current in the heat roller. As a result, the heat roller itself is heated by Joule heat on the basis of the skin effect of the roller itself.
The induction coil has customarily been formed of copper or similar highly conductive material. The problem with this kind of material is that a great high frequency current flowing through the induction coil causes the coil to heat despite high conductivity. This, coupled with the radiation heat of the heat roller, is apt to damage the insulation layer of the coil and bring about short-circuiting. There has been proposed to cause cooling air to flow through the inside of the induction coil. To enhance the cooling effect, a bobbin for wrapping the induction coil may be omitted in order to efficiently release heat output from the coil to the outside, an also proposed in the past. Although this kind of configuration, in principle, enhances the coil cooling effect to a remarkable extent, it has heretofore been considered difficult to produce. Such a scheme has not been practiced with an induction coil for an induction heating type fixing device.
As for the above coil or bobbinless coil, a mold coil is also known in the art and customarily used in, e.g., a transformer. A mold coil is sufficiently short for a given outside diameter and can therefore be implemented by a resin molding. However, when it comes to the induction coil for the heat roller having an elongate, small diameter cylindrical configuration, fluidity available with resin is too low to implement a molding.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-194399 and 9-127813.